1. Field of the Invention
The present invention relates to a method for manufacturing a transistor, and in particular to a method for manufacturing a transistor having a low leakage current.
2. Description of the Related Art
Thin Film Transistors (hereinafter referred to as TFTs) are commonly applied in active matrix LCDs and SRAMs. TFTs are classified into amorphous silicon TFTs and polysilicon TFTs. As compared to amorphous TFTs, polysilicon TFTs have a higher mobility of carriers and a greater current driving capability. However, polysilicon TFTs have greater leakage currents, disallowing their use in large area LCDs. Therefore, the reduction of leakage currents in polysilicon TFTs is an important issue.
The key factor affecting the magnitude of a leakage current is the intensity of the electric field inside the depletion region near a drain. Reducing the electric field in the depletion region results in a corresponding reduction in the leakage current. One method to reduce the intensity of the electric field inside the depletion region is to form spacers on both sides of the gate, thereby preventing the gate from overlapping with the drain. In the past, there were two ways to manufacture these spacers, which will be described along with the disadvantages thereof as follows:
(1) An additional mask is added before performing the ion-implanting to form the drain/source, thereby defining an offset region between the gate and the drain, so that the drain can not overlap with the gate. The use of an additional mask, however, results in increased complexity during the TFT manufacturing process. Furthermore, errors in mask alignment are common, impeding the precise control of spacer width. PA1 (2) A silicon oxide layer is first formed on a substrate and a gate. Then, a back etching is performed to form spacers on both sides of the gate. While this method allows precise control of the widths of the spacers, active ions used for the back etching can cause varying amounts of damage to devices, particularly gate oxide layers.
In addition to TFTs, general MOS transistors also suffer from the problem of leakage currents.